ChemicalScience

PERSPECTIVE

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The endeavor of

Key Laboratory for Advanced Materials, Fer

Center, Institute of Fine Chemicals, School

East China University of Science & Tech

200237, China. E-mail: tianhe@ecust.edu.c

Cite this: Chem. Sci., 2020, 11, 7525

All publication charges for this articlehave been paid for by the Royal Societyof Chemistry

Received 18th March 2020Accepted 5th May 2020

DOI: 10.1039/d0sc01591a

rsc.li/chemical-science

This journal is © The Royal Society o

vibration-induced emission (VIE)for dynamic emissions

Zhiyun Zhang, Guangchen Sun, Wei Chen, Jianhua Su and He Tian *

Organic chromophores with large Stokes shifts and dual emissions are fascinating because of their

fundamental and applied interest. Vibration-induced emission (VIE) refers to a tunable multiple

fluorescence exhibited by saddle-shaped N,N0-disubstituted-dihydribenzo[a,c]phenazines (DHPs), which

involves photo-induced configuration vibrations from bent to planar form along the N–N axis. VIE-active

molecules show intrinsic long-wavelength emissions in the unconstrained state (planar state) but bright

short-wavelength emissions in the constrained state (bent state). The emission response for VIE-active

luminogens is highly sensitive to steric hindrance encountered during the planarization process such that

a tiny structural variation can induce an evident change in fluorescence. This can often be achieved by

tuning the intensity ratio of short- and long-wavelength bands. In some special cases, the alterations in

the emission wavelength of VIE fluorophores can be achieved step by step by harnessing the degree of

bending angle motion in the excited state. In this perspective, we summarize the latest progress in the

field of VIE research. New bent heterocyclic structures, as novel types of VIE molecules, are being

developed, and the general features of the chemical structures are also being proposed. Technologically,

novel emission color-tuning approaches and VIE-based probes for visualizing biological activity are

presented to demonstrate how the dynamic VIE effect can be exploited for cutting-edge applications.

1. Introduction

Shedding light on the uorescent mechanisms of chromo-phores with large Stokes-shied and dual emissions is of greatsignicance, as it could provide reliable guidance for thedevelopment of high-performance multi-colour emissionmaterials.1–10 As shown in the Jablonski diagram in Fig. 1a, thedual bands of an individual uorophore can generally bedivided into small (normal) and large (abnormal) Stokes-shiedemissions.11–13 The overlap of the normal short-wavelengthemission with the lowest-lying absorption band originatesfrom the initial emissive state near the Franck–Condon excitedstate, while the red-shied emission far from the absorptionband arises from the structurally transformed emissive stateformed by additional excited-state transformations, such asenhanced charge transfer,14–19 energy transfer,20–25 complexformation,26–31 and structural and/or conformationalchanges.32–55 Due to the signicant congurational and/orstructural changes in the excited state, the ratio of short- andlong-wavelength bands can be manipulated in a controllableand dynamic manner, activating unprecedented multi-colourluminescence.9,56–61

inga Nobel Prize Scientist Joint Research

of Chemistry and Molecular Engineering,

nology, 130 Meilong Road, Shanghai,

n

f Chemistry 2020

Regarding the specic p-conjugated structures and lumi-nescent mechanisms, scientists have made notable achieve-ments with intriguing systems and concepts, such as theexcimer/exciplex,26–28,30,31 uorescence resonance energy trans-fer (FRET),20–25 twisted intramolecular charge transfer(TICT),48–55 and excited-state intramolecular proton transfer(ESIPT).36–42 Most of the mechanisms originate from the acci-dental discovery of a specic uorescence phenomenon,62–73

aer which the excited-state dynamics and the structure–prop-erty relationships were extensively studied and well established.These discoveries have been regarded as guidelines forexploring a range of applications. A self-contained lumines-cence system/concept can be constructed (Fig. 1b)50,65 thatincludes: (1) basic building blocks with common characteris-tics, (2) unique excited-state processes, and (3) amazing prop-erties and functions that have not been achieved by otherluminescence systems. A well-known example of a dual-emission system based on a unimolecular reaction is the TICTconcept,50 for which Grabowski and co-workers formulateda structural hypothesis based on the molecule N,N-dimethyla-minobenzonitrile (DMABN) in 1973.74 Donor and acceptormoieties linked by single bonds build typical TICT molecules.Upon photoexcitation, the TICT molecules are involved in rapidelectron transfer from the donor to acceptor units, accompa-nied by mutually perpendicular donor and acceptor round thesingle bond. The TICT process is highly dependent on thepolarity and viscosity of the solvent, so TICT uorophores hold

Chem. Sci., 2020, 11, 7525–7537 | 7525

Fig. 1 (a) Jablonski diagram for the normal and transformed two-state model resulting in a large Stokes shift and dual fluorescence. (b) Route forthe construction of a specific emission system and concept. (c) Illustrative schemes of two mature dual-emission mechanisms based onunimolecular dynamics: TICT and ESIPT.

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great promise in polarity, viscosity, and temperature sensing.54

Another well-known and important unimolecular type is theESIPT process,75,76 and salicylic acid was the rst reported ESIPTdye by Weller in 1955.77 Typical ESIPT molecules possess anintramolecular hydrogen bond (H-bond) between an O–H orN–H proton donor and a C]O or ]N– proton acceptor. Uponphotoexcitation, the ESIPT process involves fast proton transferfrom the proton donor to the acceptor. Generally, the ESIPTprocess can be affected by polar and H-bond donating solventsor other molecules, so the ESIPT uorophores are wonderfuloptions for uorescence sensors and imaging agents.40

Adding a new chapter to the subject of large Stokes shis anddual uorescence, we demonstrated a novel type of structure–photophysics relationship for the rst time by examination ofa set of N,N0-disubstituted-dihydrophenazines (DHPs).78,79 Thepreliminary structure N,N0-diphenyl-dihydrodibenzo[a,c]phen-azine (DPAC) and the proposed structural evolution uponphotoexcitation are shown in Fig. 2a and b, respectively. Abenzene ring and a phenanthrene ring are fused by a exiblecyclic 8p-electron ring dihydropyrazine as the junction, result-ing in a V-shaped polycyclic skeleton in DPAC, i.e., dihy-drodibenzo[a,c]phenazine. Furthermore, the considerablesteric hindrance, between the phenanthrene moiety and N,N0-diphenyl groups, results in a dramatically puckered dihy-drodibenzo[a,c] phenazine in DPAC. As a result, the twonitrogen lone pair orbitals may partially participate in the p-conjugation in the bent ground state. Therefore, the p-conju-gation, aromaticity (Baird's rule80–82) and steric hindranceeffects contribute to enhancing the geometry-bending orgeometry-attening for the ground and excited states, respec-tively. Upon photoexcitation, DPAC molecules encounter sterichindrance, producing an initially bent form (Fig. 2b), whichsubsequently overcomes the internal energy barrier and relaxesto the nal planar form with fully extended p-conjugation.79

According to Baird's rule,80–82 the bent-to-planar vibration of

7526 | Chem. Sci., 2020, 11, 7525–7537

DPAC is driven by the excited-state aromaticity reversal in thelowest excited state (S1). We termed this unconventionalabnormal large Stokes shied emission of DPAC vibration-induced emission (VIE),78,83,84 in which the photoinducedbent-to-planar transformation involves out-of-plane bending ofa V-shaped polycyclic conjugated structure.

The VIE mechanism has been utilized as a persuasiverationalization for large Stokes shis and full-colour-tunableuorescence of DPAC and its analogues under various condi-tions.84 In regular solutions (Fig. 2c), an excited molecule ofDPAC transforms the conguration from a bent state to a planarstate in S1, releasing a very weak blue emission (bent state) andprominent orange-red uorescence (planar state). However, thephotoinduced bent-to-planar transformation can be somewhathampered in highly viscous media, and the bright blue uo-rescence is emitted from the retained saddle-shaped confor-mation. Therefore, a vivid uorescence colour-tuning process ofVIE-active molecules, from orange-red to blue via white, couldbe realized by regulating the local properties (Fig. 2d). Alongthese lines, the very distinctive feature of VIE-active molecules isthat a single uorophore can exhibit a very large Stokes shiand a dynamic change in full-colour emission in the visibleregion. As an emerging class of opto-functional materials, VIE-active molecules have been utilized in thermometers,85,86

viscometers,87 chemosensors,88–90 biosensors87,91,92 and whitelight emitters.93,94 To date, cogent spectroscopic and theoreticalproofs of the excited state processes of VIE molecules have beenpresented and the structure–photophysics relationship hasbeen rmly established.84 To systematically reveal the correla-tion between the conformational and electronic variables of theVIE process, the covalent approach has been utilized to ne-tune the excited-state planarization of DPAC (Fig. 2e),95,96

producing highly colour-tunable uorescent images. Further-more, to mimic the excited-state structural evolution, thestrategy of using ortho-methyl effects has been applied to

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Fig. 2 (a) Chemical and crystal structure of N,N0-diphenyl-dihydrodibenzo[a,c]phenazines (DPAC), together with their structural characteristics.(b) Diagrammatic sketch illustrating the vibration-induced emission (VIE) involving a photo-induced configuration vibration from bent to planarform along the flexible N–N axis, driven by excited-state aromaticity reversal. (c) Steady-state emission spectra at room temperature in solventswith different polarities. (d) Temperature/viscosity dependent emission spectra ofDPAC in n-butanol from 233 K to 133 K. (e) Chemically-lockedstructures with tunable excited states. (f) ortho-Methyl effect induced tunable ground states. Reproduced from ref. 79 (c and d) and ref. 95 (e)with permission from the American Chemical Society, Copyright 2015 and 2017. Reproduced from ref. 97 (f) with permission from the Wiley-VCH, Copyright 2018.

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regulate the ground-state geometry of DPAC from a bent toa planar conguration step by step (Fig. 2f).97

This accidental discovery is turning into a sustainableachievement. In this perspective, we aim to highlight thenoticeable progress in VIE-active luminogens over the past year,including (1) novel bending heterocyclic cores for the molecularlibrary of the VIE system, (2) new strategies for dynamic emis-sion colour changes and real-time functions, (3) applications asbiological probes, and (4) excimer formation of a VIE-activeluminogen in a crystal. We also discuss the present pros andcons of the VIE system and look forward to its furtherdevelopment.

Fig. 3 (a) General characteristics of VIE-active molecules. (b) Chem-ical structures of PNO, PNS and PNSe as novel kinds of VIE-activemolecules and reference compound PNC. (c) Absorption and emissionspectra in various solvents at room temperature. Reproduced from ref.98 with permission from Wiley-VCH, Copyright 2019.

2. Designing new bendingheterocyclic cores for the VIEmolecular family

DPAC, acting as the most typical DHP core, has been widelyutilized in VIE research (Fig. 3a).84 However, such amonotonousstructure greatly restricts the development of VIE research bothfundamentally and in applications. Therefore, there is anurgent need to exploit new VIE cores. To meet this requirement,replacing the N-phenyl groups by other heteroatoms with loneelectron pairs has been the primary consideration, for which

This journal is © The Royal Society of Chemistry 2020

the aim has been to retain the central core as a exible 8p-electron ring (Fig. 3a). Additionally, according to our previousresearch on DHPs, the steric hindrance between the phenan-threne moiety and N-substituted aryl groups plays a key role inthe VIE process (Fig. 3a). Based on this perspective, a series of

Chem. Sci., 2020, 11, 7525–7537 | 7527

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DHP derivatives were designed and synthesized, in which one ofthe N-aryl groups was replaced by an O (PNO), S (PNS), or Se(PNSe) atom and dimethyl–methanediyl (PNC) (Fig. 3b).98 Asa result, these three compounds exhibited ultraviolet lowest-lying absorption bands located at 330–400 nm and stronglybathochromic shied emissions at 465, 555, and 570 nm, withcorresponding large Stokes shis of 7906, 11 480, and11 955 cm�1 (PNO < PNS < PNSe, see Fig. 3c), respectively. Theorder of the Stokes shis resulted mainly from the betterpolarization of the S and Se atoms than that of the O atom,which could extend the p-conjugation of the global minimumstate (i.e. planarization state) and thus narrow the energy gap.Notably, there was a negligible emission band at 429 nm inPNSe, which was very similar to that in DPAC, indicatinga pronounced VIE characteristic. In contrast, PNC only exhibi-ted a normal emission band at 417 nm with a Stokes shi of5313 cm�1 since the elongated p-conjugation of PNC wasinterrupted by the non-conjugated bridge CMe2. The steady-state results combined with further time-resolved and compu-tational data proved the ubiquity of VIE in DHP derivatives andprovided a sufficient theoretical basis for the development ofnew VIE cores.

By further exploring the relationship between the photoin-duced structural planarization and steric hindrance, pheno-thiazine derivatives were chosen for further study due to theirsimilar structures to those of DHPs and potential applicationsin bioimaging. Therefore, we developed a series of newcompounds NAS, NAS-1, and NAS-2 (Fig. 4a).99 According to X-ray analysis of the three compounds, the order of the bendingangles (Qa) in the phenothiazine moiety was NAS (152.08�) >NAS-1 (147.89�) > NAS-2 (131.12�). Notably, the N-phenylshowed a perpendicular orientation to the phenothiazine corein NAS and NAS-1 but a parallel orientation in NAS-2. Theseresults could safely demonstrate that the bent angleQa inNAS-2was derived from the hindrance between the naphthalene ringand N-phenyl. Further proof could also be obtained from thesteady-state spectra (Fig. 4b). NAS and NAS-1 showed coincidentonset (purple circles), which was consistent with the

Fig. 4 (a) Chemical and single-crystal structures for NAS, NAS-1 andNAS-2. (b) Absorption and photoluminescence spectra in varioussolvents at room temperature. Reproduced from ref. 99 withpermission from the Wiley-VCH, Copyright 2019.

7528 | Chem. Sci., 2020, 11, 7525–7537

characteristics of normal emission. However, NAS-2 showeda distinct separated onset (blue and red circles), which was thesame as the absorption and emission of DPAC. This separationindicated that NAS-2 possessed different congurations fromthe initial ground state to the nal S1 state. Accordingly, it wasconcluded that there was indeed a VIE phenomenon, similar tothat of DHPs, in NAS-2, which was further demonstrated by thetime-resolved data and computational approaches. This workestablished a clearer relationship between the VIE and sterichindrance and facilitated the design of VIE cores, which greatlyextended the application of the VIE mechanism.

The two studies mentioned above indicated that sterichindrance plays an important role in the dual emission in theVIE cores. However, what is the real driving force behind thephotoinduced structural planarization process?82 Such a struc-tural relaxation was reported for the conjugated 8p systemdibenz[b,f]oxepin in 1992 (Fig. 3a).33 Its preferential bentconguration in the ground state can be explained by Huckel'srule,100 in which the planar structure possessed unfavourableantiaromatic properties due to the 8p electron array in themiddle ring. Upon excitation, however, the equilibriumconguration tended to be planar, driven by excited-statearomaticity reversal, which can be explained by Baird's aroma-ticity rule.80–82 Such a structural planarization allowed for theglobal conjugation of the whole molecule and led to a largeStokes shi. Therefore, it can be speculated that the 8p electronarray is another important factor in the photoinduced structuralplanarization process. Recently, Naka et al. reported a class of8p system dibenzo[b,f]arsperins,101 which also exhibitedseverely bent boat-like congurations in the ground state(Fig. 5a). Both 3-Me and 3-Ph exhibited intrinsic emission bands(�370 nm) and longer-wavelength emission broad bands (500–700 nm), with large Stokes shis of 7249 and 6830 cm�1

(Fig. 3b), respectively. The longer-wavelength emission intensityratio was solvent dependent. It decreased with increasingsolvent polarity due to the lower polarity of the planar structurein the excited state (Fig. 3b and c). Moreover, a high viscosity

Fig. 5 (a) Chemical structures of dibenzo[b,f]oxepine and dibenzo[b,f]arsepin derivatives 3-Me and 3-Ph. (b) Single-crystal structure of 3-Ph.(c) Photoluminescence spectra of 3-Ph in various solvents. (d)Calculated potential energy surface of 3-Ph with the variation in thetorsion angle in the S0 and S1 states. Reproduced from ref. 101 withpermission from the Wiley-VCH, Copyright 2019.

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and PMMA matrix environment could also weaken the longer-wavelength emission intensity ratio, very similar to the behav-iour of DHPs, indicating that the structural planarizationprocess was impeded in such environments. Therefore, thereshould be an activation energy (DEa) in the structural planari-zation process from the Franck–Condon state to the relaxedstate, which was estimated to be 3.2 kJ mol�1. According tothese results, a typical photoinduced structural planarizationprocess can be observed in such conjugated 8p systems, whichprovides a clear direction for molecule design with the VIEproperty.

Combined with our previous work, we can determine thegeneral characteristics of VIE molecules (Fig. 3a). On the onehand, annular conjugated 8p systems provide the internaldriving forces behind the bent-to-planar process due to theiraromaticity in different states and congurations (i.e., bent/planar conguration in the ground/excited state) based onHuckel's rule100 and Baird's rule.80–82 On the other hand, thesteric hindrance between the X(Y)-substituted aryls (X(Y) is theheteroatom in the internal core) and the extended conjugatedparts of the core further enlarges the torsion angle and nelytunes the planarization process. By suitable utilization of thesteric hindrance in annular conjugated 8p systems, largerStokes shis and colour-tunable multiple emission of a singleuorophore can be achieved.

3. Dynamic emission colour change

The most distinctive feature of VIE-active molecules is that theycan achieve very large Stokes shis and dynamic emissionchanges in color.84 A preferential strategy to achieve dynamicemission colour changes is to modulate the intramolecularvibrations of DHPs and thus control their excited

Fig. 6 (a) Chemical and (b) crystal structure of DPAC-bisC4P, as well as(d) Working principle of the fluorescence tuning of DPAC-bisC4PIDacetonitrile obtained under 365 nm UV light illumination. (f) Normalizedvarious linear saturated dicarboxylates at the end of titrations. ReproducCopyright 2019.

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congurations.95,96 The supramolecular approach is an idealchoice and has been widely employed to design smart materialsdue to its facile molecular design and synthesis.102,103 Moreover,the reversible non-covalent interaction in supramolecularassemblies generates a controllable system, and thus, it mightachieve unexpected properties. As a general rule, there are twomain supramolecular approaches: (i) directly modulating theexcited conguration of the DHPs from a molecular level and(ii) taking advantage of the supramolecular assembly inducedaggregation to restrict the vibrations of DHPs.

In view of this, we developed a novel uorescent chemo-sensor DPAC-bisC4P consisting of DPAC and two appendedcalix[4]pyrrole moieties (Fig. 6a).104 Unlike conventionalsensors, the DPAC-bisC4P chemosensor could not only quan-titatively determine specic dicarboxylate dianions (DC2�) butalso differentiate certain linear saturated DC2� and threephthalate DC2� (Fig. 6c). In this work, DPAC-bisC4P could formpseudomacrocyclic noncovalent linked host–guest complexeswith various DC2� due to calix[4]pyrrole-mediated dianionrecognition, thus imposing various degrees of constraint on theplanarization of DPAC (Fig. 6b and d). In acetonitrile, anobvious red-shied emission colour change was evident as thelength of the linear saturated DC2� chains increased. Variousemission colour changes could be also observed between thethree phthalate DC2� due to different structural effects (Fig. 6eand f). According to the distinct chromaticity readouts, theseDC2� can be easily differentiated by DPAC-bisC4P, which isvividly compared to a molecular cursor calliper. This workhighlights a novel recognition-based approach for dianionsensing and even provides an effective way to achieve the facilesensing of other analytes via a rational smart design.

To explore the effect of supramolecular-assembly-inducedaggregation, two benzo-15-crown-5 parts were appended on

(c) chemical structures for various dicarboxylate anionic guests (DC2�).C2�. (e) Photographs of DPAC-bisC4PIDC2� and DPAC-bisC4P inphotoluminescence (PL) spectra of DPAC-bisC4P in the presence of

ed from ref. 104 with permission from the American Chemical Society,

Chem. Sci., 2020, 11, 7525–7537 | 7529

Fig. 7 Schematic representation of the supramolecular assembly and disassembly processes of DPAC-[B15C5]2. Reproduced from ref. 105 withpermission from the American Chemical Society, Copyright 2020.

Fig. 8 (a) Chemical structure of DPC and an illustration of the VIEprocess. (b) Mechanism of ratiometric thermometers established bythe aggregation and disaggregation of VIE-active molecules. (c) CIEchromaticity diagram, where the symbols are themeasured values andthe line segments are the calculated analogous values. (d) The fittedcurves of the CIE coordinates corresponding to the FL spectra of DPCin an ethanol/glycerol mixture with fg ¼ 40% at different temperatures.Reproduced from ref. 106 with permission from the Royal Society of

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the DPAC moiety at the para site of 9,14-phenyl through estergroups to form a potassium ion (K+) responsive molecule DPAC-[B15C5]2 (Fig. 7),105 in which DPAC was the uorescent signallerand benzo-15-crown-5 was the K+ recognition site. 15-crown-5could form a 2 : 1 sandwich-like supramolecular assemblywith K+. Meanwhile, the diameter of K+ (2.66 �A) was so muchsmaller than the distance of the centres of the two 15-crown-5moieties in DPAC-[B15C5]2 (23.18 �A) that only an intermolec-ular supramolecular assembly could be formed, thus causingaggregation of the system and constraining the planarization ofthe DPAC moiety. By ne tuning the degree of aggregation, theemission colour changed from orange-red via white to blue withthe gradual titration of K+. Conversely, a reversible processcould clearly be seen in which the emission colour recoveredfrom blue via white and orange-red with the titration of 18-crown-6, because 18-crown-6 possessed a higher complexingability with K+ and thus caused disaggregation of the system.Moreover, water was found to have a similar effect on 18-crown-6. Even trace amounts could generate evident emission colourchanges in the system. In the utilization of this supramolecularassembly, the detection limit of water in toluene was calculatedto be as low as 0.035 wt&. This work provided sufficient prooffor and a deep understanding of the relationship amongsupramolecular interactions, aggregation, and the VIE process.

Temperature is another important factor that inuences themolecular vibrations, which makes DHPs an ideal candidate forthe construction of thermometers.85,86 To be specic, on the onehand, a lower temperature provides less energy for the thermalmotion of DHPs, and may keep them in bent excited states evenif they are in the form of a single molecular state. On the otherhand, a low temperature tends to reduce the solubility of DHPsin solution and cause aggregation, thus restricting their excitedstate planarization process. However, only ultra-low tempera-tures (below �233 K) can suppress the vibration of DHPs innormal solutions, which severely limits their application in theeld of daily temperature detection. Therefore, the dilemmabetween daily temperature requirements and the vibrationrestriction of DHPs must be overcome. In this regard, wedeveloped an ultra-sensitive and range-tunable ratiometricthermometer DPC (Fig. 8a).106 Its excited bent-to-planar processcould be tuned via temperature-induced aggregation anddisaggregation. Based on the different solubilities of DPC inethanol/glycerol mixed solvents at different temperatures andglycerol fractions, the degree of aggregation could be

7530 | Chem. Sci., 2020, 11, 7525–7537

controlled. An increasing temperature would dissociate theaggregates and enhance the orange-red emission, whilea decreasing temperature would re-aggregate the dissociatedDPC molecules and enhance the blue emission, therebyachieving a signicant ratiometric response to temperature(Fig. 3b). Moreover, the range-tunable property could be real-ized by simply tuning the fraction of each component of themixed solvent, since ethanol is a good solvent for DPC whileglycerol is a poor one. As a result, it was realized for the rsttime that this uorescent ratiometric thermometer possessedan ultra-high sensitivity at �2000% �C�1 and a broad overalllinear response region from �11.4 �C to 37.7 �C. More inter-estingly, the uorescent signal at various temperatures exhibi-ted a good linear relationship on the CIE chromaticity diagram(Fig. 8c and d). In this manner, temperature information couldbe invaluably read with only a smartphone instead of a profes-sional spectrometer. This work perfectly resolved the conictbetween high sensitivity and broad response range andprovided a novel perspective for developing thermometryschemes.

Chemistry, Copyright 2020.

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In addition to using solvents as the media, DHPs incorpo-rated into polyurethanes (PUs) can be applied not only fortemperature detection but also for exploring the microphaseseparation of PUs.107 PUs possess particular properties due tothe thermodynamic incompatibility between the so and hardsegments, thus making it possible to tune the excited intra-molecular vibrations of the DHPs. An alternative strategy is tointroduce DHPs into PUs through physical blending or chem-ical bonding. Therefore, we used DPAC doped in PUs (PU-4701-1) and its derivatives graed to PUs in different forms (PU-4701-2, 3, 4). As shown in Fig. 9a, the DPACmoieties were external tothe PU-4701-1 chain and subjected to little restriction in theplanarization process. Consequently, they exhibited pinkemission colours at room temperature. In the case of PU-4701-3,however, DPAC was graed to PUs via bilateral chemical bonds,which greatly suppressed the excited bent-to-planar process andemitted strong blue uorescence. As for PU-4701-2 and PU-4701-4, the unilateral chemical bonds between DPAC and PUsimposed weaker restrictions than PU-4701-3, but stronger than

Fig. 9 (a) Diagrammatic sketch illustrating the DPAC-doped polyurethtemperatures (�30 to 90 �C). (c) Scattering curves of PU-4701-1 achieveReproduced from ref. 107 with permission from the American Chemical

This journal is © The Royal Society of Chemistry 2020

PU-4701-1, thus emitting pink-purple uorescence. Tempera-ture is an important factor in the change in the microphasestructures of PUs. For example, PU-4701-2 exhibited a cross-regional emission colour change from blue to orange-red inthe region of �30 �C to 90 �C (Fig. 8b). The high consistencybetween the continuous temperature cycles and the small-angleX-ray scattering experiments provided an alternative scheme formeasuring the degree of microphase separation in the PUs asa “uorescent ruler” (Fig. 8c). This work opens up a new path forpolymer investigation through uorescence techniques.

4. Biological probes

VIE-active compounds can act as ideal signal moieties becauseof their unique dual emission properties, which is benecial forachieving ratiometric detection.108–111 Hence, developing VIE-based probes for visualizing biological activity is extraordi-narily fascinating.87,91,92 The enzymatic uorescent probe Q1based on the VIE mechanism was designed and synthesized for

anes PU-4701-1–PU-4701-4. (b) PL spectra of PU-4701-2 at variousd by small-angle X-ray scattering at various temperatures (10–90 �C).Society, Copyright 2019.

Chem. Sci., 2020, 11, 7525–7537 | 7531

Fig. 10 (a) Synthesis of VIE-based sensorQ1. (b) Diagrammatic sketch illustrating the working principle ofQ1, showing VIE properties before andafter reacting with phosphatase in living cells. (c)–(e) Fluorescence images of LO2 and HeLa cells. Reproduced from ref. 112 with permission ofthe Royal Society of Chemistry, Copyright 2020.

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detecting and imaging the protein tyrosine phosphatase 1B(PTP1B) in living cells (Fig. 10a).112 Q1 was composed of theDPAC moiety and a phosphate radical and dissolved well insolvent, thereby exhibiting a free emissive state. When Q1reacted with PTP1B, hydroxyl-substituted DPAC was acquiredand showed poor solubility, thereby exhibiting a xed emissivestate and enhanced blue emission (Fig. 10b). Unlike theconventional VIE phenomenon, this probe showed almostconstant orange-red emissions at �600 nm during the enzy-matic reaction process with PTP1B, which could act as an“internal standard”. However, the increasing blue emissionintensity could be applied for detecting PTP1B as the reaction

Fig. 11 (a) Molecular structures of DPAC, DCM26 and DCM23. (b) Schemsensor for the ratiometric fluorescence sensing of influenza viruses. (cinfecting influenza viruses (IVs) into host cells. (d) Fluorescence imagesificities. Reproduced from ref. 113 with the permission of Elsevier, Copyr

7532 | Chem. Sci., 2020, 11, 7525–7537

proceeded. Through this mechanism, Q1 was determined tohave a detection limit of 5.7 ng mL�1 for PTP1B, and it showeda rapid response, and good selectivity and stability. Further-more, Q1 could be used to detect PTP1B in living cells(Fig. 10c–e). Under the conditions of constant emissions in thered channels, Q1 emitted brighter emissions in the greenchannel in HeLa cells, indicating that HeLa cells containedmore PTP1B. This work provided an idea for the construction ofVIE-based biological probes.

Hydrophobic DHPs are a series of ideal molecules fordeveloping core–shell structures in the eld of biosensors.Based on this viewpoint, He and co-workers developed

e for the stepwise self-assembly to prepare a core–shell sialyllactosyl) Illustration of the sensor utilized for blocking the entry of human-of DPAC/DCM26 sensor in the presence of IV with different HA spec-ight 2019.

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a sialyllactosyl probe for inuenza virus (IV) detection andblocking.113 DPAC acted as the core structure and theaggregation-enhanced blue uorophore due to its poor solu-bility and restricted conguration in aqueous solutions. DCM-based sialyllactosides were applied to assemble with DPAC toform a sialyllactosyl probe, emitting a red uorescence viaForster resonance energy transfer (FRET) from DPAC to DCM(Fig. 11a and b). Since the specic binding blocked the FRETprocess, a strong DPAC channel (blue) could be observed whilethe DCM channel (red) was nearly absent when the probe wastreated with H3N2 or H7N9. However, the non-specic colli-sions between H10N8 and the probe particles had little inu-ence on the dual emissions, thus exhibiting only a strong DCMchannel (Fig. 10d). Apart from the sensing ability, the probecould also suppress the IVs entering human cells by antago-nizing the interactions between the viruses' surface-boundhemagglutinin and cell-surface sialylglycans (Fig. 11c). Thisstudy provided further understanding of the construction ofbiological probes through the smart design of DHPs by utilizingtheir aggregation-enhanced uorescence property.

5. Excimer formation of VIE-activeluminogen in a crystal

Excimer emission26–28,30,31 and emission mechanochrom-ism114–117 have been found in the crystalline solids of the VIE-active compound 12-phenyl-12H-benzo[a]phenothiazine (NAS-2, see Fig. 4a and 12).99 Normally, NAS-2 crystals are expected toemit normal Stokes-shied emissions due to the lattice energy,which could prohibit the planarization process. However, theNAS-2 crystal exhibited dual emissions in the crystal state,located at 390 and 520 nm (Fig. 12a). Compared to the emissionof the DPAC crystal located at �386 nm, it is believed that the

Fig. 12 (a) Photoluminescent spectra of NAS-2: black solid line for the gcrystal with different depths from the surface to the kernel. Packing oluminescence characteristics ofNAS-2 in the crystal asmonitored at varioVCH, Copyright 2019.

This journal is © The Royal Society of Chemistry 2020

short-wavelength emission originated from the monomer bentconguration of NAS-2. Since the planarization process result-ing in the long-wavelength emission was excluded due to thelattice energy, intermolecular interactions can be considered tobe the main factor, most likely as an excimer. This speculationwas rst proven by molecular packing in the crystal (Fig. 12band c). The different bending angles of NAS-2 and NAS-1 lead todifferent orientations of N-phenyl due to the steric hindrance.As a result, NAS-2 is more suitable for forming proximal p–pinteractions for the naphthalene unit, while NAS-1 avoids anypossible p–p stacking. By investigating the emission behaviourof the NAS-2 crystal based on the depth from its surface, therewas mainly a 390 nm emission (purple-blue) at the surface, anddual emission (pale blue) at the kernel of the crystal(Fig. 12a and d). Hence, it was concluded that excimer forma-tion was preferred as the depth increased, while p–p stackingruptured at the surface due to the dangling molecules.Furthermore, the existence of the excimer was also proven bythe transient photoluminescence (Fig. 12e). The decay compo-nent of 20.8 ps monitored at the normal emission of 400 nmwas well matched with the rising component of 21.3 ps moni-tored at the excimer emission of 500 nm. The discovery of theexcimer in NAS-2 is signicant, opening up an avenue for thestudy of the p–p interactions of VIE-active luminogens.

Broadly speaking, the role of excimer or dimer is one of themost important issues for organic luminogens in the solid state,which has been explored in the eld of room-temperaturephosphorescence (RTP).118–120 Utilizing VIE design principlesto achieve tunable RTPmay be feasible. To achieve this, it is veryimportant to understand the actual role of the p–p interactions.In this regard, Tang, Li, and co-workers provided a clear direc-tion by exploring the effect of the dimer in persistent RTP indetail.121 By adopting the phenothiazine 5,5-dioxide group as

round powder, and coloured solid lines for the red square in (d) of thef (b) NAS-2 and (c) NAS-1 in the crystal lattice. (e) Transient photo-us wavelengths. Reproduced from ref. 99 with permission of theWiley-

Chem. Sci., 2020, 11, 7525–7537 | 7533

Fig. 13 (a) Issue for the function of the dimer in the persistent room-temperature phosphorescence (RTP) effect and the proposedhypothesis, in which F1 denotes fluorescence from the monomer(singlet state, S1), P1 denotes RTP from the monomer (triplet state, T1),F2 denotes fluorescence from the excimer (S1), and P2 denotes RTPfrom the excimer (T1). (b) The chemical structure of CS-2COOCH3

together with the design strategy. Reproduced from ref. 121 withpermission of the Royal Society of Chemistry, Copyright 2020.

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the donor and dimethyl isophthalate as the acceptor, a newcompound, CS-2COOCH3, revealed the origin of all the photo-luminescence species, including the persistent RTP, singletexcimer and thermally activated delayed uorescence (TADF)emission (Fig. 13). Upon excitation, both the singlet excitonsand excimers showed delayed uorescence emissions withdifferent wavelengths and lifetimes. However, only one mole-cule was involved in the natural transition orbitals of the tripletstate for the dimer, despite strong p–p interactions (Fig. 13).Therefore, the persistent RTP was a monomer-dominatedemission species, with the p–p interactions weakening thephosphorescence radiative and non-radiative decay. As for thetriplet excimer, there was too low a radiative rate to exhibitphosphorescence. This study was signicant for achievinga deep understanding of the RTP process. Without beinglimited to p–p interactions, a clear insight into the excited stateelectron transition in various photophysical processes greatlycontributes to extending the applications. Our research alongthis line based on VIE cores is also in progress, and it is ex-pected to further our understanding of the relationship betweenVIE and other photophysical processes.

6. Summary and perspectives

In this perspective, we have reviewed the recent achievements inthe design of signicant VIE-active luminogens. Deep insightinto the photoinduced conformation planarization has beengained by tailoring the new bending heterocyclic cores for theVIE molecular family. Such brilliant molecular engineering of

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the VIE system clearly reveals the ubiquity of the VIE process inDHP derivatives. It also proved that the annular conjugated 8punits provided the internal driving forces behind the photoin-duced bent-to-planar process, while the steric hindranceenlarged the torsion angle and nely tuned the planarizationprocess. Thus, stimuli-responsive multiple emission systemshave been achieved by functionalized VIE-active molecules,which endowed them with precise ratiometric uorescencedetection for supramolecular polymer thermometers, as well asbio-probes.

Despite the great progress in VIE studies over the past veyears, there are abundant challenges to broadening the funda-mentals and applications of VIE territory. A major taskremaining is to develop various new VIE cores. The maximumemission wavelength of the current VIE-active luminogensbased on phenazine, phenothiazine, phenoxazines or pheno-selenazine cannot reach the near infrared (NIR) region, whichseverely restricts the biological applications of the VIE concept.A common strategy to solve this issue is to extend the p-conjugation of the core by utilizing larger polycyclic skeletons orappending other conjugated systems. Furthermore, it is knownthat such annular conjugated 8p systems are only a particularcase that achieves the VIE process. According to Huckel's ruleand Baird's aromaticity rule, the aromaticity of the internal ringis the driving force behind the photoinduced structural vibra-tions. Therefore, designing larger annular systems withincreasing numbers of conjugated electrons should be analternative method to achieve NIR emission or even unexpectedresults.

Likewise, water-soluble VIE-active molecules are in greatdemand, since most of the existing ones can only be well dis-solved in organic solvents, which also greatly limits their bio-logical applications. The majority of the reported VIE-based bio-probes do not exhibit an obvious emission colour changebecause of the poor orange-red emission in aqueous environ-ments. It has been reported that appending polar functionalgroups, such as quaternary ammonium and carboxyl groups, ona VIE core can facilitate water solubility, providing a practicablemethod for molecular design. In addition, graing VIE-activeprobes on water-soluble macromolecules, such as poly-ethylene glycol (PEG), polysaccharides and peptides, may notonly improve the water solubility but also instil goodbiocompatibility.

Another important issue is to explore the relationshipbetween VIE and other dynamic processes, such as electron andenergy transfer, which are probably the most ubiquitous light-initiated pathways. Further in-depth insight into the competi-tion between the VIE process and electron/energy transfer cangreatly advance our knowledge of the time scale of such anexcited state conguration transformation and reveal someunknown problems. Most importantly, these complicateddynamic processes could mimic VIE-based bio-probes in themembranes of organelles, such as mitochondria and chloro-plasts, because both the viscosity and the redox/energy transferreaction, corresponding to our situations of planarization, PETand FRET respectively, can be probed by the emission of VIEmolecules. That is to say, VIE-based bio-probes could measure

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two parameters simultaneously in biomembranes, providinga correlation between the membrane viscosity and the efficiencyof cellular respiration or photosynthesis. It is inevitable thatsystematically studying the excited state process of complexsystems will be of great signicance both in terms of scienticvalue and in practical applications in the future. The nal goalof ‘dynamic living molecular emitting probes for living cells’would be realized based on this VIE-concept molecular design.

As an expanding eld, VIE research simultaneously providesus with opportunities and challenges. We hope this perspectivewill provide guidance for the innovation of smart VIE-activelight-emitting materials and facilitating applications in theeld of bio-sensors and bio-imaging.

Conflicts of interest

There are no conicts to declare.

Acknowledgements

We acknowledge the nancial support by NSFC/China(21788102, 21905090, 21790361, 91859205), Shanghai Munic-ipal Science and Technology Major Project (Grant No.2018SHZDZX03), the Programme of Introducing Talents ofDiscipline to Universities (B16017), Shanghai Science andTechnology Committee (17520750100). Z. Y. Zhang thanksProgram for Eastern Scholar at Shanghai, and Natural ScienceFoundation of Shanghai (19ZR1412200).

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